The present invention relates to the photoconducting detector in optical immersion, sensitive to infrared radiations, comprising a lens with convex face transparent to infrared radiations; on the side of the lens opposite its convex face a wafer of semiconducting material of one of the two types of conductivity N and P; and, between the lens and the wafer, a layer for assembling the lens and the wafer.
A detector of this type is already known, in particular by U.S. Pat. No. 2,964,636.
Such detectors are at present produced with a lens in CdTe, or Ge, and a detecting wafer in Hg.sub.1-x Cd.sub.x Te, which is an alloy of CdTe, a material with wide forbidden band, and of HgTe, a semi-metal material and whose wave-length of detection depends on the value of x. The detectivity of these detectors, defined by the signal-to-noise ratio and which is representative of their performances, is limited, in certain cases, by their thermal noise or their capacitive noise, without being influenced by the ambient background and its fluctuations. These infrared photodetectors not limited by the ambient background are called non-BLIPs (background limited infrared photodetectors). They are used in particular in the military field for the evaluation of targets or for telemetry.
To increase the level of the signal whilst maintaining constant that of the noise, is the same as increasing the detectivity. This is what is obtained by optical immersion, by increasing the apparent surface of the detector proper.
This apparent surface may be multiplied by n.sup.2, n representing the index of the lens, if the detector is placed at the centre of a hemispherical lens, or by n.sup.4, if the detector is placed at the first Weierstrass point of a hyperhemispherical lens, extending beyond the centre of the corresponding sphere.
As to the detectivity itself, it is multiplied by n in case of hemispherical immersion and by n.sup.2 in case of hyperhemispherical immersion, if it is limited only by the thermal noise, and by n.sup.2 in case of hemispherical immersion and by n.sup.4 in case of hyperhemispherical immersion, if it is limited only by the capacitive noise.
Finally, optical immersion makes it possible to produce detectors operating at high temperature or high electrical frequency, and presenting higher detectivities than those of the non-immersed detectors under the same conditions.
In fact, and inversely, for a determined apparent sensitive surface, the real surface and therefore the thermal noise may be decreased, i.e. the detectivity may be increased.
To produce these photodetectors, the following procedure was heretofore carried out: a detector made elsewhere was purely and simply glued on a lens, the assembling layer evoked hereinabove being a layer of glue.
However, this layer of glue presents drawbacks.
The glue is an intermediate medium of low refraction index, which limits, by total reflexion, the field of sight of the detector. Moreover, it is not easy to choose a glue which simultaneously satisfies the requirements of index, infrared radiation transmission in the spectrum in question, resistance to the thermal cycles, coefficient of heat expansion, mechanical solidity, rate of degassing, stability, chemical inertia, etc . . . .
It is therefore an object of the present invention to propose a detector of the type referred to above but not presenting the afore-mentioned drawbacks.